Construction workers were 25 feet underground excavating a concrete vault for a Hetch Hetchy water system seismic upgrade in Fremont and weren’t expecting to find artifacts at the site. Then came the bison forearm relic, followed by about an additional 50 Ice Age-era fossils.

Science

“We knew it was something fairly large that we were finding,” said Jim Walker, project paleontologist for the San Francisco Public Utilities Commission.

Archaeology is a slow business. Walker and the SFPUC crew continued to excavate the muddy ground for a year after the August 2013 discovery. In May, the public will get a chance to see those remnants of the past, which were donated to the Children’s Natural History Museum in Fremont, when an exhibit opens.

“As we looked, we found more round boulders and bone fragments, and then we started to find some other pieces that were clearly different,” Walker said. “We realized we had more than one animal in there.”

It is still unclear what many of the fossils are, Walker said, but they reveal important details about the evolution of the Bay Area. For example, the bones were found buried under huge, round boulders, indicating that Fremont was once crisscrossed by fast-moving streams and covered in lush megafauna. The bones caught and collected in spots along the waterways.

“Some of them were broken apart during transfer and when they settled, making them hard to identify,” he said. “It’s a puzzle. You have to run through databases to figure out what these bones are. It’s a fun mystery that, when solved, can help us understand important impacts the environment has on us.”

An area of grasslands

Many of the fossils date to the Rancholabrean period, about 11,000 to 240,000 years ago during the ice age. Back then, the East Bay looked much like the Serengeti. Grasslands stretched to the horizon, broken only by trees, brush and fast-moving streams. The Hayward Fault had not yet pushed up mountains and foothills, and the bay was empty of water.

Native horses, bison, sloths, camels, Columbian mammoths and saber-toothed tigers roamed the area. All of the species later went extinct — today’s horses descend from those brought by European settlers.

The remains of at least two bison, brush rabbit, pocket gophers, horse, elk, deer and camel were found from this era at the dig. Another layer of fossilized fish, crayfish, mussels, freshwater snails, amphibians and reptiles date to the Irvingtonian North American Land Mammal Age, about 240,000 to 1.8 million years ago, and indicate the expanse was once covered by a fresh water lake.

“We weren’t really expecting to find fossils,” said Betsy Rhodes, regional communications manager for the SFPUC. “Amid all the high-tech construction, there was excavation happening 25 feet down. As we dug further down, we found about 50 specimens from the Ice Age down in the vault, which was unexpected but very fun.”

Photo: Connor Radnovich, The Chronicle

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Fossilized bones at the Children's Natural History Museum in Fremont, California, on Thursday, Dec. 31, 2015.

Fossilized bones at the Children's Natural History Museum in Fremont, California, on Thursday, Dec. 31, 2015.

Math/Science Nucleus board president Joyce Blueford at the Children's Natural History Museum in Fremont, California on Thursday, Dec. 31, 2015.

Math/Science Nucleus board president Joyce Blueford at the Children's Natural History Museum in Fremont, California on Thursday, Dec. 31, 2015.

Photo: Connor Radnovich, The Chronicle

Exciting Ice Age fossil find in Fremont; exhibit to open in May

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Readying the exhibit

Preparations for the fossil’s public debut are under way. At the Children’s Natural History Museum, Joyce Blueford, a retired geologist and board president of Math Science Nucleus, the nonprofit that oversees the museum, sorted through the fossils on a folding table. Baggies of bones and vials of invertebrates cover the surface, and shiny, 12,000-year-old mussels perch on a black cloth.

The display has not yet been finalized, and labeling the different species is the biggest hurdle.

“This is a leg joint, possibly for a bison,” she said, picking up a lumpy, fossilized bone. “This bison was about 20 percent bigger than its modern-day cousins. The animals started getting smaller and smaller because the environment became more limiting over time. These aren’t as sexy as the big mammoths, but they can still tell us a lot about glaciation and climate changes.”

Bison a key find

The biggest discovery in the batch was the bison, Blueford said. Their introduction signals the end of the mammoth and other large mammals. She ran a finger along the humerus bone, the one originally found at the site, pointing out scratches and bite marks from scavengers.

“These fossils are important because they tell a story of the evolution of the Bay Area,” she said. “Everyone knows about dinosaurs, but we have Ice Age fossils, too. They tell us about the evolving landscape and climate. This is the beginning of a story interpreting what happened in the past.”

The PUC has finished construction of the Hayward Fault pipe crossing. No further relics are expected to be found, and any left in the depths will remain mired in their final resting place.